Kelsier really wasn't kidding when he said that. Be very wary of spoilers for all of the Mistborn books.

“ There's always another secret.
”

— Kelsier's mantra

Kelsier is a half-skaaMistborn from Scadrial.[5] He is stern and intense but always smiling; another act of rebellion against the Lord Ruler's oppression. He hates injustice and takes this hatred to the extreme, which originally resulted in him being unable to see good in any of the nobility. This was changed by Vin who fell in love with Elend Venture. Kelsier went on to save Elends life from an inquisitor. His late wife Mare collected pre-Ascension memorabilia, and he keeps one piece, a flower[6], to inspire his crew. Kelsier is the younger brother of Marsh, who is disappointed at Kelsier's apparent lack of direction or purpose until Kelsier leads the skaa rebellion. He always keeps a smile on his face, believing that is the best way to fight.

Kelsier is tall and hawk-faced and has light-blonde hair and hazel eyes[7][8]; he has a multitude of large scars crossing both arms from his time in the Pits of Hathsin.[9] He usually wears a nobleman's suit (A colored vest with dark trousers and a coat) and for night time activities his mistcloak.

He is almost always late to any gatherings, which he attributes to always having somewhere better to be[10]. He always smiles and projects an excited and positive attitude, which is particularly unusual in the skaa underground. He does this in order to fight the Lord Ruler's oppression of all the skaa, he wants to have one thing which cannot be taken away from him.[9] He hates all of the nobility and tends to believe that all of them are to blame for the plight of the skaa, although near the end of his life this attitude changes due to Vin's insistence that Elend is different and that not all the nobles should be fought. He has an imposing presence and is a natural leader, easily taking charge and swaying even the downtrodden skaa of Luthadel into joining the rebellion with his charming personality. However he is not above using this personality to manipulate those around him to his own ends, as can be viewed through his spreading of the rumours of the "eleventh metal".

Some might say that Kelsier is a psychopath, though this is debatable due to the intense anger he exhibits when innocents (or rather his idea of innocents) are harmed, which is a very empathetic trait. He also shows regret and guilt for what happened to Vin in the aftermath of the Kredik Shaw burglary. Throughout the series he has fond memories of his wife whom he misses greatly, and her death caused him to snap into Allomancy. Nevertheless, Sanderson states that Kelsier derives pleasure from killing people, but only allows that part of his personality to surface occasionally, when he is fighting noblemen, and he tries to channel this aspect of his personality into a good cause.[11] This leads us to believe that he isn't a psychopath by definition,he is most likely insane in another way.

Born to a noble father and skaa mother[12], his mother kept both Kelsier and Marsh alive by pretending that she was a noble to their father so that their children would be considered merely illegitimate noblemen instead of half-breeds; however, their father eventually found out. Thus Kelsier's mother was killed, and because of this, the brothers have a deep hatred of noblemen. Kelsier then made his living as a thief, becoming famous throughout Luthadel. However, after a failed attempt at stealing from the Lord Ruler himself in his secret room inside Kredik Shaw, he was sent to the atium mines in the Pits of Hathsin[13]. There he spent his time believing that Mare had betrayed him to the Lord Ruler, until he witnessed his wife being beaten to death by guards after she gifted her Atium bead to him. He then "Snapped", becoming a Mistborn[6]. Using Allomancy he was able to escape the Pits. Kelsier was then trained by a mistborn named Gemmel[14] and they travelled to the far north of the Western Dominance to a city called Mantiz where Kelsier heard rumours of the Eleventh metal from the skaa in Keep Shezler. Kelsier sought and eventually found the Eleventh metal as he believed it would help him to defeat the Lord Ruler.[14]

After discovering the Eleventh metal Kelsier travels back to Luthadel and takes a job from the skaa rebellion to bring down the final empire. After gathering a team of some of the best (and Kelsier believed, the kindest) thieves in the city, including skilled mistings, he sets in motion a plan to destroy the Final Empire and kill the Lord Ruler. After saving her from her former crew, he takes on Vin as a student after confirming his suspicions about her being a Mistborn and saving her from Camon[7]. While pulling off the plan Kelsier himself takes on many roles, acting as a beacon to the repressed skaa he assists in recruitment[15], he attacks many Noblemen to increase the tensions between the Great Houses[16], he collects information by purchasing it from informants and also poses as an informant himself in order to provide misinformation to Noblemen.[17] Though they suffer many setback, Kelsiers leadership allows the rebellion to attain an army of sufficient size to hold Luthadel.

When several skaa from the rebellion are about to be executed, Kelsier attempts to save them. He is confronted by a Steel Inquisitor which he eventually manages to defeat by beheading him with the Inquisitor's own obsidian axe. However, Kelsier is then confronted by the Lord Ruler, who backhands him across the face and then stabs him in the chest with a spear, killing him.[2] Kelsier later reveals in his letter to Vin that he had intended to use the Eleventh Metal to destroy the Lord Ruler but could not discover how to use it in his fight.[18] His death results in the founding of the Church of the Survivor, who call him "Lord of the Mists". As a result of this Vin also becomes a major religious figure.

Kelsier was met by Preservation directly after his death. He then asked him if he was God, and after being told yes he promptly punched him in the face.[19] Then he finds the power at the Well of Ascension and becomes trapped there with Ruin, allowing him to remain in the cognitive realm.[19] While he is in the Well, Hoid passes by and Kelsier essentially challenges him to a fight that Kelsier ultimately loses.[20] He feels Ruin's presence during his time in the Well.

Once he is freed from the Well along with Ruin, he initially travels around through the cognitive version of Luthadel, seeking out his friends. He finds Vin and Ham near the steps of Keep Venture and also learned that Dockson had died in the Battle of Luthadel.[21] He walks past Luthadel towards the lake of the same name, near which he finds Khriss and Nazh.[22] They question him on Preservation's appearance after learning he has spoken directly with the shard, and then continue their discussion onto other topics. Khriss mentions the Ire which sets Kelsier onto his next quest of finding their base of operation.[23]

During his journey to find the Ire, he runs into Ruin and has a discussion about Ruin's intent.[4] After that, he left the area of the Cognitive Realm surrounding Scadrial and moved closer to finding the Ire. When he reaches their base, he moved past their guards and passed the wall into their chambers.[24] He ultimately only took a glass orb from them after thwarting their plan to take over being the vessel of Preservation.[25]

When he returns to Scadrial he finds that Preservation's power is significantly weakened. He also runs into Vin and is shocked to see how far she has come in the time he was away.[26] He stays with her for a while after that, but ultimately has to leave. [27]

When Leras died and lost power over the shard of Preservation, Kelsier took the power and held it for Vin on behalf of Leras.[28] While he held the power, he found Spook and told him to send a message to Vin that any spiked person could potentially be under Ruin's control.[29] He also continually clashed with Ruin during his time holding the shard Preservation.[30][31] When Vin's spike is removed, Kelsier relinquished Preservation's power to her.[31] After she and Elend are killed, Kelsier speaks with them before they pass on into a different realm.[32]

After Sazed's ascension and Spook's rise to political power, Kelsier begins to speak with Spook about finding a way to bring himself back to the Physical Realm.[33]

An apparition with Kelsier's features appears to Spook, giving him the ability to burn pewter, but this is later revealed to be Ruin, who is using Kelsier as a guise.

After Spook removed his pewter spike, Spook hears Kelsier's actual voice, telling him that they were friends, since Kelsier was able to 'piggyback' on Preservation's power after its death. [35] Kelsier appeared and told Spook in a vision about Hemalurgy and how ruin can speak to people pierced by a Hemalurgic spike during the coma Spook had following his flooding the canals of Urteau and escaping the fire at the Canton of Inquisition, right before he woke up. For a time, Kelsier could only speak to Spook since Spook was the only one who believed in Kelsier as a deity, since Kelsier was a deity for a brief time, between Preservation's death and the coming of the Hero of Ages. [36] After Marsh removed Vin's earring, Kelsier was able to speak to her. [37]

“From the Well of Ascension, of course. It's the same power, after all. Solid in the metal you fed to Elend. Liquid in the pool you burned. And vapor in the air, confined to night. Hiding you. Protecting you.Giving you power!
”

In the second generation of the series, the cult which started after Kelsier's deception with OreSeur directly following his death has become a fully-formed religion. The Church of the Survivor, or Survivorism, is possibly the most organized of Scadrial's religions in this period, as well as being the clear favorite among the social elite and downtrodden alike. Steris and Marasi are both members of this religion, though Marasi at least admits that it is more because she was raised in it. Its central tenet is simply to survive, by any means, until you cannot survive any longer. Priests in this religion wear robes with stitching up the arms to emulate Kelsier's scars from the Pits of Hathsin. The "Survior's Statemark" is one of the symbols of the faith. When Father Bin was killed by Bleeder, reference is made to the mockery of the Survivor's Statemark in how he was killed.[42] The Statemark may have reference to an eye or eyes pierced with a hemalurgic spike.

In The Bands of Mourning a character with features matching Kelsier's makes his first tangible appearance in the post-Final Ascension era. During a scene in which Wax taps a coppermind deftly hidden in the coin he was given by a seeming beggar in New Seran, he sees a vision through the eyes of a man who sees half as normal men do, and half in the steel-sighted vision of a Steel Inquisitor. Finding a small village of Southern Scadrians, ancestors of Allik Neverfar, the man approaches the freezing people, reaches out with a scarred arm, and commands them to "Survive," Kelsier's own commandment. This would imply that Kelsier was the "Sovereign" Allik had referred to, the one whose statue had held the spear on which the Bands of Mourning were discovered by Wayne.

Kelsier is a Mistborn, so he can burn every Allomantic metal, although he is particularly proficient with the external physical metals iron and steel[2]. He is capable of using them with a high degree of precision to control the momentum and rotation of dozens of pieces of metal simultaneously. He lacks finesse in the external mental abilities of zinc and brass as he tends to create large changes in emotional states as opposed to the more subtle Soothing abilities of Breeze, although he can affect a relatively large mass of people.[15] He is quite proficient at pewter dragging and has enough practice to maintain it for hours and still maintain a fair amount of clarity afterwards.[43]

“I decided that I’d see her dream fulﬁlled. I’d make a world where ﬂowers returned, a world with green plants, a world where no soot fell from the sky...." He trailed off, then sighed. "I know. I’m insane.
”

—Kelsier to Vin, after showing her the picture of a Marewill flower[6]

“If you’re always on time, it implies that you never have anything better you should be doing.
”